Add mount dust particle effects

Implemented dust cloud particles that spawn at mount feet when running
on the ground, similar to the original WoW. Features:

- Brownish/tan dust particles using point sprite rendering
- Spawn rate proportional to movement speed
- Particles rise up, drift backward, and fade out naturally
- Only active when mounted, moving, and on ground (not flying)
- Smooth particle animation with size growth and alpha fade

Uses similar particle system architecture as swim effects with
GL_POINTS rendering and custom shaders for soft circular particles.

CMakeLists.txt

@@ -143,6 +143,7 @@ set(WOWEE_SOURCES
     src/rendering/minimap.cpp
     src/rendering/world_map.cpp
     src/rendering/swim_effects.cpp
+    src/rendering/mount_dust.cpp
     src/rendering/loading_screen.cpp
     src/rendering/video_player.cpp